Optical disk data storage systems provide the capability to store large quantities of data on a disk. The data is accessed by focusing a laser beam onto the data layer of the disk and then detecting the reflected light beam. Various kinds of systems are known. In a ROM (Read Only Memory) system, such as a compact disk system (CD-ROM), data is permanently embedded as marks in the disk at the time of manufacture of the disk. The data is detected as a change in reflectivity as the laser beam passes over the data marks. A WORM (Write-Once Read-Many) system allows the user to write data by making marks, such as pits, on a blank optical disk surface. Once the data is recorded onto the disk it cannot be erased. The data in a WORM system is also detected as a change in reflectivity. There are also erasable optical data storage systems, such as phase change and magneto-optic (M-O) systems. While phase change systems also read data by sensing a change in reflectivity, M-O systems read data by measuring the rotation of the incident polarization caused by the M-O media. In all of these systems the focusing lens for the optical beam is located away from the disk.
The data density in optical disk drives is determined by the size of the recorded marks or pits on the disk, which is limited by the diameter of the focused laser spot on the disk. This spot size is the same as the diameter of the focused optical beam, also called the beam waist size. The waist size of a focused light beam is given approximately by .lambda./2NA, where .lambda. is the wavelength and NA-nsin.theta. is the numerical aperture of the lens. The waist size can be reduced by either using shorter wavelength lasers, such as blue lasers, or by employing higher NA lenses.
One approach to achieving a high NA lens is to use a lens of high index of refraction (n) material and to position the lens close to the recording layer on the disk. Such a high NA lens is a solid immersion lens (SIL) which is made in the shape of a hemisphere and is described by S. M. Mansfield et al., Optics Lett., vol. 18, p. 305, 1993. As shown in FIG. 1A, the hemispherical SIL has a thickness r, where r is the radius of the sphere. An air gap separates the flat surface of the hemispherical SIL and the surface of the optical disk. The NA is increased by a factor of n due to the wavelength reduction in the lens.
U.S. Pat. No. 5,125,750 proposes an optical disk drive with a conical section of a hemispherical SIL supported on a conventional air-beating slider of the type used in magnetic recording disk drives. The conical section of the hemispherical SIL is held within the slider by springs so that the flat surface of the SIL faces the disk surface. Because this approach is limited to using only a small conical section of a hemisphere as the SIL, the realizable NA and thus the spot size is severely limited. This approach has the additional disadvantage that there is no continuous air-bearing surface (ABS) on the slider because the SIL projects through the slider body with the flat surface of the SIL forming part of the ABS. It is therefore difficult to make the flat surface of the hemispherical SIL so that it is in exactly the same plane as the ABS. If the SIL protrudes through the ABS the slider will not fly properly and the SIL will touch the disk, damaging both the disk and the SIL. If the SIL is recessed from the ABS by even a fraction of a wavelength, the optical resolution will be degraded.
Another type of SIL is known. In this SIL, referred to here as a "super-hemispherical" SIL and shown in FIG. 1B, the lens includes a partial spherical section and has an overall lens thickness greater than the r thickness of a hemispherical SIL. The super-hemispherical SIL has a thickness r(1+1/n), where r is the radius of the partial spherical section. A focused spot will be obtained at the base of the lens when the incident rays are converging toward a point located a distance nr below the center of the sphere. The incoming converging rays are refracted at the surface of the partial spherical lens section, resulting in an increased effective incident angle .theta.. In the super-hemispherical SIL, the NA is increased by a factor of n.sup.2, as compared with an increase by a factor of only n in the hemispherical SIL.
In both the hemispherical and super-hemispherical SILs the small spot exists only within the high index of refraction material because the high angle rays will be internally reflected at the base of the SIL. However, these rays can be coupled via their evanescent fields to the optical disk if the disk is placed less than a wavelength distance from the base of the SIL. Thus the SIL can be used to increase the storage capacity of an optical disk by the square of the spot size reduction, but only if the SIL can be kept much closer than one wavelength from the disk.
What is needed is an optical disk drive with a super-hemispherical SIL supported on an air-beating slider that is maintained consistently within one wavelength from the disk and that has a continuous ABS.